Passive or Active Intake

[Tokensmoke10]

My grow cabinet is just over 9 cubic feet and was thinking about setting up a passive intake.

As you can see i took off the bottom board that went down to the floor so now there's an opening about 2 in off the floor that runs the front length of the box. This allows me the option of having a passive intake.

I've only heard good things about passive intakes but I'm unsure about the specifics. I'm going to be using around 250 watts of CFL's. But I still have a few questions.

1.To keep the floor of the cabinet still usable, would it be better to go with several smaller holes opposed to 1 or 2 large ones?
2. How big/how many exhaust fans would I need for a space of 9 cubic feet, I was thinking 3 80mm axial fans or 2 120mm axial fans.

Thanks everyone
2.

 

[Stakkur]

Personally i'd go with several smaller holes rather than few bigger ones, just try and place them so that they go between pots not under them.
As for the fans its the CFM rating you are intrested in rather than their actual size, though i'd go with fewer and larger since they spin at a slower speed than the smaller for the same airflow and thus generate less noise.
If i recall correctly about 1 CFM will keep 3 wats of CFL about 5 degrees above the exterior temp, tho that migth be way off.

 

[420warriors]

You want be able to change the air once a min (9CFM fan or a little bigger) and for that small of a space I'd go passive. If you put to many holes or to large the cab won't maintain a negative pressure (more CFM out than in). That may not matter so much for cooling purposes but is very important if you're using a carbon scrubber, a negative pressure will keep air and more importantly, smell from escaping the room. I learned this from one of my heating and a/c friends, and think it is usually overlooked in passive systems. Sorry but I don't know how to figure out the CFM of the holes, but if they are too small it will put too much load on the fan and shorten it's life or just burn it up.

 

[Tokensmoke10]

Couldn't you just figure out the sq in of the holes for exhaust fans then figure out how many sq inches you would need to be twice that of exhaust. Then divide that by how many passive intakes you want and that's the size of each intake. Right?

 

[420warriors]

The hole size has nothing to do with how much air the fan moves though. For example look at bathroom vent fans, you can get one from 50CFM to 120CFM and use 4" vents. I just tried to google a formula to figure CFM and came across a computer site that said for cooling, a positive pressure is better (more intake air than exhaust) . so I guess maybe I was wrong. But maintaining negative pressure is important when exhausting to remove odor, that way odor all goes through the scrubber and not out any unsealed areas.

 

[Tokensmoke10]

Interesting, it's weird how you can read about both ways being more cooling efficient. l know I've read that negative pressure forces heat up and out the exhaust better than an active intakes. I honestly don't know which one to believe

 

[420warriors]

I know what you mean,really as long as you're bringing in fresh air it has to be helping. that other site also mentioned the intake of dust which is bad for the electrical components in a computer but don't apply to our application. But it made me wonder if a piece of foam or something would help keep bugs out.

 

[Stakkur]

You can just glue a thin piece of cloth over the intake, that should work as a filter on most bugs/dust